var stream=require("stream");
var Readable=stream.Readable;
var util=require("util");

function MyReadable(params) {
    Readable.call(this);     
}
// for(var i in Readable.prototype){
//     MyReadable.prototype[i]=Readable.prototype[i];
// };
util.inherits(MyReadable,Readable);
var m=new MyReadable();

var c='a'.charCodeAt(0);
m._read=function (params) {
    // console.log(c);
    if(c<'z'.charCodeAt(0)){
        m.push(String.fromCharCode(c++));
        //rs._read遇到rs.push结束
    }else{
        m.push
    } 
}
m.pipe(process.stdout);